The letter said: "It is clear that the energy market is broken and urgently needs fixing.

"Top of our concerns is the need to increase competition and to make trading transparent.

"For too long the lack of competition in the energy market has not been addressed. It is now time for radical changes that deliver an effective, competitive market that works for everyone, before the scale of this crisis worsens."

The energy regulator Ofgem is expected to announce its intention to refer the energy suppliers to the Competition and Markets Authority this week.

Such a referral could ultimately lead to a break-up of the companies, forcing them to separate their power generation arms from their retail businesses.

In response, a spokesman for Energy UK said: "The energy industry supports competition.

"The increasing number of companies looking to supply energy, allied to the numbers of customers switching and choosing smaller suppliers, shows that the market is active and working in the interests of customers."